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Differential Detection of Alternatively Spliced Variants of Ciz1 in Normal

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Differential Detection of Alternatively Spliced Variants of Ciz1 in Normal Rahman et al. BMC Cancer 2010, 10:482 http://www.biomedcentral.com/1471-2407/10/482 RESEARCH ARTICLE Open Access Differential detection of alternatively spliced variants of Ciz1 in normal and cancer cells using a custom exon-junction microarray Faisal A Rahman1,2, Naveed Aziz1, Dawn Coverley1* Abstract Background: Ciz1 promotes initiation of mammalian DNA replication and is present within nuclear matrix associated DNA replication factories. Depletion of Ciz1 from normal and cancer cells restrains entry to S phase and inhibits cell proliferation. Several alternative splicing events with putative functional consequences have been identified and reported, but many more variants are predicted to exist based on publicly available mRNAs and expressed sequence tags. Methods: Here we report the development and validation of a custom exon and exon-junction microarray focused on the human CIZ1 gene, capable of reproducible detection of differential splice-variant expression. Results: Using a pair of paediatric cancer cell lines and a pool of eight normal lines as reference, the array identified expected and novel CIZ1 splicing events. One novel variant (delta 8-12) that encodes a predicted protein lacking key functional sites, was validated by quantitative RT-PCR and found to be over-represented in a range of other cancer cell lines, and over half of a panel of primary lung tumours. Conclusions: Expression of CIZ1 delta 8-12 appears to be restricted to cancer cells, and may therefore be a useful novel biomarker Background replication in late G1 phase nuclei [5] and that it coop- Ciz1 (Cip1 interacting zinc-finger protein 1) is a nuclear erate with cyclin E and A in order to execute its func- protein that appears to be encoded by 17 exons that tion in DNA replication [2]. Further detailed analysis map to chromosome 9q34 in man (Unigene Hs.212395). revealed that DNA replication activity resides in the Ciz1 is expressed in a wide range of tissues [1] and con- N-terminal half of Ciz1, while the protein becomes teth- tains N-terminal polyglutamine repeats, 3 zinc finger ered to non-chromatin nuclear matrix structures motifs, one matrin 3 domain at the C-terminal end, 2 through C-terminal sequences that play a role in its putative nuclear localization signals and a number of localization to DNA replication factories [6]. Taken cyclin-dependent kinase phosphorylation sites and together the data suggest that Ciz1 plays a linker role cyclin-binding motifs [2]. Ciz1 was first identified between the DNA replication machinery and the sub- through its interaction with p21/Cip1 [3], a cyclin- nuclear structures that organize their function. dependent kinase (CDK) inhibitor involved in regulation CIZ1 is a hormone-responsive gene [7,8] and is thought of the cell cycle and in cellular differentiation. We iden- to play a role in the development or progression of oestro- tified Ciz1 in a cell-free system that reconstitutes initia- gen responsive tumours. It has also been linked with cer- tion of DNA replication, under the regulation of cyclin tain paediatric cancers [1,9], where alternatively spliced A/CDK2 [4]. Using cell-free and cell-based approaches transcripts are prevalent. Similarly, inappropriate expres- we showed that Ciz1 stimulates initiation of DNA sion of alternatively spliced transcript has been reported to occur in brain tissue from Alzheimer patients [10]. Pre- viously, we surveyed over 600 CIZ1 expressed sequence * Correspondence: [email protected] tags that map to the CIZ1 Unigene cluster [9], to overview 1Department of Biology, University of York, UK Full list of author information is available at the end of the article alternative splicing during development and disease. This © 2010 Rahman et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Rahman et al. BMC Cancer 2010, 10:482 Page 2 of 12 http://www.biomedcentral.com/1471-2407/10/482 highlighted three exons in the 5′ end of Ciz1 that are com- include probes homologous to exons, observed junc- monly alternatively spliced and potentially mis-spliced tions, hypothetical junctions and retained intron under some circumstances, and suggested a range of other sequences (Fig. 1). The array also includes probes rarer splicing events. homologous to other genes that either emerged from In this study our aim is to look at CIZ1 variant our biochemical studies as Ciz1 interacting partners, expression in order to validate previously reported alter- have roles in cell cycle predicted to interact with Ciz1 native splicing (AS) events, and to discover new ones. or are known to be involved in pathology of sporadic We chose to use an exon junction array because com- cancers (additional file 1 Table S1). For 5′ untranslated monly used methods for splicing analysis such as sequences we included probes that cover possible alter- RT-PCR, cDNA primer extension and nuclease protec- native first exons. These are defined as exon 1a (in tion mapping are extremely labour intensive for highly NCBI Reference Sequence: NM_012127.2 and spliced genes, particularly when analysis of a large num- NM_001131015.1) and exon 1c (NM_001131017.1, ber of samples is expected. Therefore, we have devel- NM_001131018.1 and NM_001131016.1). Other alterna- oped an exon junction-array focused on the human tively used exon 1 s were predicted by AceView (addi- CIZ1 gene that is capable of detecting all known tional file 2 Figure S1) and confirmed by us when variants and hypothetical variants of CIZ1. We present searching CIZ1 EST transcript against the BLAT CIZ1 results generated with this exon array-tool, initially assembly [11]http://genome.ucsc.edu/, and designated using a pair of paediatric cancer cell lines. This identi- exon 1b and 1d (additional file 3 Figure S2). To our fied several novel AS events including one cancer- knowledge inclusion of exon 1b and 1d has not been restricted novel variant that we chose to validate further. verified by inclusion in a full-length human CIZ1 The functional implications of expression of this variant transcript. (which has partial deletion of exon 8 and exon 12 and To allow for a reasonable trade off between signal skipping of exon 9, 10 and 11) are discussed and poten- intensity and specificity, we restricted length of the tial applications highlighted. probes to 40 nucleotides [12]. To increase specificity of probes, interspersed repeats and low complexity DNA Methods sequences were masked (repeats have been replaced by Array and probe design Ns) using repeat masker [13]http://www.repeatmasker. We designed oligonucleotide probes with extensive cov- org. The masked sequences were then searched for simi- erage for CIZ1 mRNAs and predicted variants. These larity against BLAT and areas which showed significant Figure 1 Schematic representation of the location of human CIZ1 probes, designed to detect expression of the most common predicted exons, less common alternative exons, rare alternative exons containing intronic sequences, most common and observed rare exon-junctions and a selection of hypothetical junctions. For junction probes between most common predicted exon, - indicates the junction such as Ciz1ex2-ex3. For junction probes between less common predicted exons, such as Ciz1 exon 2 alternative 1 and exon 2 alternative 2, the probe designation will be defined as Ciz1-ex2a1-ex2a2. Probe sequences are given in additional file 1 Table S1. Rahman et al. BMC Cancer 2010, 10:482 Page 3 of 12 http://www.biomedcentral.com/1471-2407/10/482 similarity were excluded. For junction probes, we European Collection of Cell Cultures (ECACC) and the employed the sliding window method where the 40 nt Japanese Collection of Research Bioresources (JCRB). can slide across the target junction to allow for near Cells were cultured exactly as recommended. Total similarity in melting temperatures [12]. Probes were RNA was extracted using TRIZOL as recommended by then searched for similarityusingBLASTnon-redun- the manufacturer (Invitrogen). RNA concentration was dant database with expectation value of 10 http://blast. quantified using nanodrop spectrophotometer and equal ncbi.nlm.nih.gov/Blast.cgi[14]. Probes with more than 20 concentrations from each of the eight samples were contiguous identical bases to non-self transcript were pooled together. SKNMC and TC466 Ewing tumour cell excluded. For all probes, exclusion criteria were set to lines were used as test samples to validate the array and include those with hairpin structures with stem exceed- were handled as described previously [9]. ing 9 bases as well as those with self complementarities of more than 6 bases [15]. However, due to the con- Preparation and labeling of templates straints of the exon junctions, these parameters could 20 μg of pooled RNA reference control or test samples not be applied to all probes. As the CIZ1 gene is rich in (SKNMC and TTC466) were reversed transcribed as GCs, the target Tm was set not to exceed 85°C (addi- described and labeled using Invitrogen protocol (Super- tional file 1, Table S1). Probes were synthesized by Script ™Plus Indirect cDNA labeling System). Labelled Operon Biotechnologies GmbH, Cologne, Germany. The reference and test sample (20 μl each) were combined oligonucleotide probes were printed on Nexterion Slides in an amber tube, mixed and heated at 99°C for three E(Schott)usingaQArrayMini (Genetix) as follows: minutes before hybridization. 5 replicates were printed for each probe in a random pattern including buffer as negative controls. Two slides Prehybridization and Hybridization from each batch of printing were analysed for spot qual- Prior to hybridization slides were prepared as mentioned ity using Ribogreen (Molecular Probes). Prior to hybridi- previously. Hybridization was carried out in a final sation, each slide was washed at room temperature as volume of 40 μl in Corning hybridization chambers, in a follows: 1 × 5 min in 0.1% Triton X-100, 2 × 2 min in water bath at 42°C for 16-24 h.
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